When a permanent break occurs in one of the branches of a polling loop 2 wire cable, the devices which are affected are easily identified and enunciated by the security system control making it relatively simple for a service person to locate and repair the break. However, if a permanent short circuit or severe overload condition occurs on the polling loop, all of the devices on that loop can no longer communicate to the system's control, rendering the security system inoperative. Locating and repairing a break in the polling loop is much simpler than locating and repairing shorts or overloads on the loop, especially when the short or overload manifests itself only when polling loop power and signals are applied to the loop.
Present commercial security systems which use an Ademco polling loop, advertised as Vplex, periodically test the loop to detect a short or overload condition as it may occur somewhere on the loop, but they cannot locate where the trouble condition exists. The control's present reaction is to remove power from the loop and, after some time delay, re-apply power to the loop and re-test for the short or overload condition. The control has no course of action other than report the problem and continue to test the loop until the short or overload condition is removed by a service person.